1. Field of the Invention
The present invention relates to a reflector arrangement for use with a radiation source to provide high intensity, uniform and specular illumination of an exposure plane. The term "specular" as used herein means "collimated" or "substantially collimated", i.e. comprised of radiation in which the rays have a small angle of divergence.
2. Description Relative to the Prior Art
The present invention is concerned with the difficult problem of uniformly illuminating a relatively large area exposure plane with high intensity, specular radiation. More specifically, the present invention relates to a reflector arrangement which, when used with a radiation source positioned as taught by the present invention, provides illumination of a type that is useful in the photographic replication of a video record device, such as a video disc, by means of optical contact printing. In order to appreciate the problems involved in providing such illumination, it is useful to examine the photographic replication of video discs in more detail.
In theory, replicate video discs can be mass produced from a master video disc by (1) placing the master and replicate discs in intimate contact and (2) exposing the replicate disc through the master disc with actinic radiation. In actual practice, however, this seemingly simple contact printing process has not yet found widespread application among prospective video disc manufacturers. Instead, a more complex replication process involving sophisticated embossing techniques is generally used wherein a master die (which contains the video information in the form of billions of micron sized bumps) is prepared and pressed into contact with deformable plastic material which becomes the replicate disc. There appear to be at least three reasons why the simpler contact printing process is not used:
(1) There is a problem in finding a photographic replicate medium in which billions of closely spaced micron sized data bits can be photographically recorded. PA0 (2) Even if such a photographic replicate medium were available, it is difficult to maintain intimate contact, between master and replicate, over the relatively large area involved (the standard video disc is 30 cm in diameter). PA0 (3) Lastly, it is difficult to illuminate such a large area with radiation of sufficient intensity, uniformity and specularity to make the contact printing operation successful.
U.S. patent application Ser. No. 715,017 filed Aug. 16, 1977 of common assignee concerns the first two problems. (U.S. application Ser. No. 715,017 was refiled as Continuation Application Ser. No. 891,865 on March 30, 1978, now abandoned.) Briefly, that application discloses a photographic replicate medium having certain properties which enable the billions of closely packed, micron sized data bits to be recorded with excellent quality, even if intimate contact is not achieved between the master and replicate devices during the contact printing process. The third problem, i.e., providing proper illumination of the master disc during contact printing, remains, however, and has posed serious obstacles to a commercially acceptable contact printing replication process for video discs. This problem is further complicated by the fact that at least one of the materials disclosed in U.S. Ser. No. 715,017 has been found to release nitrogen gas upon exposure to actinic radiation. The nitrogen gas is released in such quantities that if one attempts to replicate in such materials by optical contact printing, the master and replicate devices are forced apart during the exposure. It may be that if the exposure duration can be made sufficiently short, the exposure can take place before the nitrogen gas forces the sandwiched master and replicate devices out of contact. All other factors constant, decreasing the exposure time means that the intensity of the exposing sources must be increased. It has been found, however, that--due to the relatively slow photographic speed of the replicating material--even a high intensity xenon flashlamp is not sufficient, by itself, to provide illumination of sufficient intensity to make such short exposure times feasible.
It is known to use certain types of reflectors as a means of altering the intensity distribution of a radiation source (such as a xenon flashlamp). For example, one such reflector theoretically produces parallel rays of light from a light source located at the focal point of a parabolic reflector. In actual practice, the light source is of finite extent so most parts of the source are not precisely located at the focal point, and divergence of rays from a common optical axis is produced. Further, light emitted from the source in a direction away from the parabolic reflector (and away from the target area) is lost. This unused radiation can amount to a substantial, even major, portion of the total light energy.
Another form of reflector is an elliptical reflector with a light source positioned at the near focal point. The elliptical reflector focuses the light from the source into the far focal point of the "ellipse". The finite extent of the source prevents perfect focusing, the result being that the light rays are concentrated in a small but finite area about the far focal point. Elliptical reflectors are, therefore, generally used to provide high intensity illumination of small areas. This type of reflector is inherently unsuitable for optical contact printing of video discs because of the relatively large disc areas which must be illuminated.
Both of the above types of reflectors, and all other reflectors having a curved reflective surface, use optical power to accomplish focusing of light rays emitted from a source. The optical power is provided in the curved reflective surfaces. The illumination pattern of such a reflector is very sensitive to the uniformity and positioning of the source, as well as to the precise shape of its curved surface. The result is an effect, sometimes referred to as "zoning", wherein shadows, hot spots, or other types of uneven illumination occur in the exposure plane. With preferred video disc replication materials, non-uniformities of illumination can cause loss of video information or a reduction in the signal-to-noise ratio. Thus, the above discussed zoning problems make curved-surface reflectors generally unattractive for use in replicating video discs by optical contact printing.
In an effort to avoid these problems, it has been suggested that a multi-faceted reflector could be used which contains no curved surfaces. (Proceedings of the Microelectronics Seminar, San Diego, Calif., 1974, Oct. 21-22, Lenses And Optical Systems Used In Microelectronics by Robert E. Hopkins, Tropel, Inc., p. 37). As taught in that paper, a light source is positioned inside a pyramidal reflector to provide a light source for a projector. The source is positioned within the reflector such that the reflective facets produce an apparent source comprised of an annular array of virtual images of the true light source. As stated in the paper, "[t]he ring of light has to be treated carefully for it is exactly the shape of image that the optical system doesn't want as far as resolution is concerned . . . [I]t would be necessary to introduce some scattering to throw light into the center of the aperture to avoid the doughnut-shaped image of light that it forms." While these comments are directed to the use of such a reflector in a projection optical system, they apply as well to optical contact printing, i.e., use of the pyramidal reflector as taught in the above paper would still produce a doughnut-shaped angular distribution of illuminance, and the annular array of source images would decrease the specularity of illumination, thereby lowering the resolution of the contact printing process.
Thus, while some type of reflector appears necessary to provide high intensity, uniform and specular illumination for the contact printing replication of video discs, both curved surface reflectors and multi-faceted reflectors, such as the pyramidal reflector described above, are unsuitable because of inherent non-uniformities in illumination and/or decreases in the specular quality of illumination.